[brimble]

A ton of countries would join NATO ASAP (if allowed in, and several would be) or rush to form new collective defense coalitions that include at least one other nuclear power, which in many cases would likely come with significant concessions to that nuclear power (i.e. China gains a de facto, if smallish, empire overnight).

Everyone refuses to trade with Russia until they're reduced to one of the most miserable countries on the planet. If their nuke program falls apart, several countries take some territory from them and no-one minds. Possibly they're reduced to selling parts of the country (maybe on a lease-like arrangement, which isn't unprecedented) in exchange for re-opening limited trade with countries that are willing, but see that they really have them over a barrel (China, again, is a likely beneficiary)

Several countries start or re-prioritize nuclear programs. Turkey, Iran, South Korea, and maybe even Japan, all likely candidates. Maybe more.

That's roughly what I'd expect the world to do about it, if the result isn't a spiral into outright nuclear war, or a swift and decisive coup in Russia. No, it's not good, but it's especially not good for Russia.